In the manufacture of optical waveguides a fiber is typically drawn from a symmetrically heated, rod-shaped, glassy preform with the fiber yielding a faithful replica of the preform in cross section. This is done by feeding the preform downwardly into a furnace and drawing the fiber from molten material formed at the bottom of the heated preform. The fiber is then passed downwardly through a coating applicator where a thin layer of coating material is applied to the fiber. This coating serves to prevent airborne particles from impinging upon and adhering to the surface of the just drawn glass fiber itself which would weaken it. The coating also shields the fiber from surface defects inflicted by subsequent manufacturing processes and installation handling. The fiber is then routed through a curing oven where the coating is cured and then onto a takeup reel via a system of sheaves and capstans. For a more detailed description of such a fiber drawing process reference may be had to the article titled "Drawing Lightguide Fiber" by David H. Smithgall and Daryl L. Myres that appears on pages 49-61 of the Winter 1980 issue of THE WESTERN ELECTRIC ENGINEER.
In conducting the just described fiber draw process it is important that the fiber be moved precisely along a preselected path. Deviations from this path may result in the fiber actually contacting an edge of the relatively small opening at the bottom of the furnace, which opening is purposely made quite small to minimize the intake of ambient air or gases. Furthermore, deviations from the preselected path through the coating applicator may well result in the coating not being applied concentrically to the fiber. In other words, the coating may be thicker to one side or the other. If the coating is badly nonconcentric the fiber may be directly exposed to ambient atmosphere; even where the coating is only marginally nonconcentric fiber alignment for connectorization becomes poor creating significant transmission losses at interconnect points. In addition, where a diameter gauge is employed in the draw machine it too cannot operatively tolerate substantial misalignment of the fiber.
The difficulty in maintaining the movement of the fiber along a preselected path beneath the glassy fiber preform arises primarily from the fact that the point of emergence of the fiber from the necked down molten mass at the bottom of the preform tends to wander or drift. A prime reason for this drift is the non-straightness of the starting preform.
Heretofore, little has been done to alleviate the fiber guidance problem just described. As a static alignment measure optical fiber preforms, that have been manufactured with some degree of arc, have been straightened by heating the preform to its softening point locally and mechanically straightening the preform with graphite paddles. Such straightening does tend to center the point of emergence better as the preform is consumed in the drawing process. However, this method may introduce additional flaws on the surface of the preform due to contact with the graphite paddles.